The preferred embodiments relate to wireless communication systems and, more particularly to minimum-shift keying (MSK) and offset quadrature phase shift keying (OQPSK) communications.
MSK is a particular instance of frequency shift keying, meaning each binary value is modulated by a respective different frequency, that is, a binary data value of 0 is communicated by a sinusoid at a first frequency and a binary data value of 1 is communicated by a sinusoid at a second frequency. Specifically in MSK, the two different frequencies provide a continuous phase, in that the difference between the first and second frequencies is equal to half the data rate. Because MSK transitions between 0 and 1 (or 1 and 0) have no discontinuity in phase, MSK is resistant to non-linear distortion and abrupt transitions between 0 and 1 (or 1 and 0) are avoided, thereby likewise avoiding the potential drawbacks that could arise from such disruptions (e.g., sideband interference to other channels).
OQPSK is a version of quadrature phase shift keying (QPSK), where QPSK communicates two binary values, sometimes referred to as a symbol, at a time, where the potential binary variation of each of the two values may represent any one of four different combinations (i.e., 22=4)—each respective combination, therefore, is represented by one of four different quadrature (i.e., 90 degree apart) values. Moreover, for a given symbol, its two bits are separated (e.g., demultiplexed) based on the odd and even positions of the bits in a continuous group of bits, with the odd bit position typically referred to as the I bit (in-phase) and the even bit position typically referred to as the Q bit (quadrature phase). Each of the I bit and Q bit is multiplied times a respective but 90 degree apart sinusoid, for example with the I bit multiplied times a cosine wave and the Q bit multiplied times a sine wave, and the resultant waves are then added to form a summed sinusoid representing the I/Q symbol. Further, due to the orthogonality of the sine and cosine mix, the two concurrently-communicated bits do not interfere with one another and the phase of the sinusoid therefore represents (e.g., can be demodulated to provide) the binary values of the I and Q bit values that gave rise to the phase. Thus, for a sequence of symbols S0, S1, . . . Sn, if symbol S0 is represented by a first sinusoid phase corresponding to bits I0 and Q0, then if only one or the other of I0 and Q0 changes value for the next successive symbol S1, the phase shift between S0 and S1 is 90 degrees (i.e., π/2), whereas if symbol S0 is represented by a first sinusoid phase corresponding to bits I0 and Q0, then if both I0 and Q0 change values for the next successive symbol S1, the phase shift between S0 and S1 is 180 degrees (i.e., π). In the offset version of the QPSK modulation technique, however, that is, for offset QPSK or OQPSK, then QPSK as discussed above is changed by delaying the Q bit by one-half of a symbol period (i.e., one bit period) relative to the I bit. Thus the range of phase transitions is 0 degrees and 90 degrees, that is, the possibility of a phase shift of 180 degrees is eliminated, so amplitude fluctuations that otherwise could occur from a 180 degree phase shift are eliminated, thereby lessening amplitude change intensity.
Sin-shape offset quadrature phase shift keying (sin-shaped OQPSK) is a special case of OQPSK in which the I and Q baseband signals are shaped by a half-sine function. The main benefit of this technique is that the resulting waveform has a constant envelope. Constant-envelope modulations are advantageous since they allow a simpler architecture of the transmitter power amplifier stage and the ability to run at higher efficiency levels, where efficiency is the relationship between the consumed power from the rail to the transmitted RF power.
Given the above-described attributes of the MSK and OQPSK modulation techniques, network transceivers have been constructed for each type of modulation. In this regard, a typical OQPSK receiver, equipped with a matched filter, has a better sensitivity performance than a typical MSK receiver. On the other hand, an MSK receiver, utilizing an FM demodulator, is a very simple and low-cost design, so in many cases an MSK receiver is used due to its low complexity and as a low-cost solution by trading off sensitivity performance as compared to OQPSK.
By way of further background, FIG. 1 illustrates a block diagram of a partial signal path in a prior art OQPSK transmitter (or transceiver), indicated generally at 100. In transmitter 100, information bits are input to an FEC (forward error correction) and interleaver block 112. Note that information bits are intended to be the raw data for communication, sometimes referred to baseband data, and which can represent various types of information, such as voice, image, control, and other data types. FEC and interleaver block 112 adds FEC correction to the data, and interleaving as known in the art re-arranges the ordering of data (e.g., rows to columns) so as to further enhance signal-to-noise (SNR) strength.
The output of FEC and interleaver block 112 is a binary stream, Rn, which is input to a bit differential encoding (BDE) block 114. BDE block 114 is included in the prior art so as to overcome some of the error spreading risk described below. Particularly, BDE block 114 modifies the input stream, to provide a corresponding output stream, En, according to the following Equation 1:En=Rn⊕En−1  Equation 1Thus, BDE block 114 creates in its output an exclusive OR of an input bit with the immediately-preceding output bit.
The output binary stream, En, is connected as an input to a spreading block 116, which converts (or spreads) each binary value into a spread sequence of N (e.g., N=8) bits. The term “chips” may refer to each of these individual bits coming out of the spreading block 116, which therefore are shown and hereafter referred to as chips QPSKn. Note that a “chip” may in itself be a bit (can only adopt two binary values) but this term is defined here to easily refer those bits coming from the output of spreader block 116. Spectrum spreading is a widely used technology to enhance a system's receiving sensitivity performance, so notably in connection with the lesser sensitivity performance of an MSK receiver, spectrum spreading has presented an approach, and meanwhile spreading also may be implemented in OQPSK transceivers. In one approach to spectrum spreading, a binary value is represented by a spreading sequence having a respective plurality (e.g., eight) of bits, so that a first binary value, and its respective set of bits, is more readily distinguishable from a second binary value, and its respective set of bits. For example, in the OQPSK domain, the mapping for respective prior art spreading sequences OQPSKPA0 and OQPSKPA1 for a binary value of 0 and 1 are as shown in FIG. 2, as Table 1. Note in FIG. 2 that the Hamming distance, that is, the number of bit positions at which the corresponding values are different, is eight. In other words, the prior art spreading sequence OQPSKPA0 for binary value κ is entirely the bit-by-bit complement of the prior art spreading sequence OQPSKPA1 for binary value 1. Such an approach, therefore, enhances signal recovery in an OQPSK receiver.
The spread chips QPSKn are input to an OQPSK modulator 118, which modulates the bits as an OQPSK waveform signal, that is, according to the quadrature technique described above. Moreover, where the modulator is a sin-shaped modulator, it will apply a pulse function, as shown in the following Equation 2, across two chip periods, that is, for a chip period of TC, the applied pulse interval is across 2TC, as follows:
                              p          ⁡                      (            t            )                          =                  {                                                                                          sin                    ⁡                                          (                                              π                        ⁢                                                  t                                                      2                            ⁢                                                                                                                  ⁢                                                          T                              c                                                                                                                          )                                                        ,                                                                              0                  ≤                  t                  ≤                                      2                    ⁢                                                                                  ⁢                                          T                      c                                                                                                                                            0                  ,                                                            otherwise                                                                        Equation        ⁢                                  ⁢        2            
While the above prior art approaches have served useful in various applications, as earlier noted there also are tradeoffs in the better OQPSK sensitivity performance versus the simplicity and lower cost of MSK. Thus, the present inventors seek to improve upon the prior art, as further detailed below.